1. Field of the Invention
The present invention relates to a backlight unit, and more particularly, to a backlight unit for slimmed structure and exhibiting high brightness through the improvement of its luminance efficiency and the quality in appearance of which is improved by the removal of bright lines, and a liquid crystal display device having the same.
2. Discussion of the Related Art
A cathode ray tube (CRT), one of the generally used display devices, has been mainly used in television receivers (TV) and monitors of measuring instruments or information terminals. However, the CRT has a relatively heavy weight and a large size. Thus, the CRT cannot satisfy the current demands of small size and light weight.
Hence, flat panel display devices, such as a liquid crystal display device (LCD) using an electric field optical effect, a plasma display panel device (PDP) using a gas discharge, and an electro luminescence display device (ELD) using an electric field luminous effect, have been proposed to substitute for the CRT device. Among them, much research has been carried out on the LCD.
The liquid crystal display device is much slimmer and lighter than any other display devices, and it requires less power consumption. Recently, the liquid crystal display device has been developed to such an extent that the liquid crystal display device sufficiently serves as a flat panel display device, with the result that the liquid crystal display device has been used in monitors of desktop computers and large-sized information display devices as well as monitors of laptop computers. For this reason, the demand for the liquid crystal display device is continuously increasing.
The liquid crystal display device is a non-emissive display device that displays images through the adjustment of the amount of light incident from the outside, and therefore, the liquid crystal display device needs an additional light source for irradiating light to an LCD panel, i.e., a backlight unit.
Generally, the backlight unit, used as the light source of the liquid crystal display device, may be classified as a direct-type backlight unit or an edge-type backlight unit based on the arrangement of cylindrical fluorescent lamps.
The edge-type backlight unit is constructed in a structure in which a lamp unit is mounted at the side of a light guide plate for guiding light. The lamp unit includes a lamp for emitting light, lamp holders fitted on opposite ends of the lamp for protecting the lamp, and a lamp reflection plate surrounding the outer circumference of the lamp, while one side of the lamp reflection plate is fitted into the side of the light guide plate, for reflecting the light, emitted from the lamp, toward the light guide plate.
The edge-type backlight unit, constructed such that the lamp unit is mounted at the side of the light guide plate, is applied to relatively small-sized liquid crystal display devices, such as monitors of laptop computers and monitors of desktop computers. The edge-type backlight unit exhibits a high light uniformity and durability, and the edge-type backlight unit is advantageous to the slimness of a liquid crystal display device to which the edge-type backlight unit is applied.
On the other hand, the direct-type backlight unit has been intensively developed with the increase in size of the liquid crystal display device to more than 20 inches. The direct-type backlight unit is constructed in a structure in which a plurality of lamps are arranged at the bottom of a diffusion plate in a line to directly irradiate light to the front of an LCD panel.
The direct-type backlight unit exhibits a higher optical efficiency than the edge-type backlight unit. For this reason, the direct-type backlight unit is mainly used in a large-sized liquid crystal display device requiring a high level of brightness.
However, a liquid crystal display device adopting the direct-type backlight unit, used in large-sized monitors or large-sized television receivers, is normally used for a longer period of time than a liquid crystal display device adopting the edge-type backlight unit, used in laptop computers. Furthermore, the liquid crystal display device adopting the direct-type backlight unit has a larger number of lamps than the liquid crystal display device adopting the edge-type backlight unit. As a result, a possibility of some lamps not lighting due to breakage and death of the lamps is stronger in the liquid crystal display device adopting the direct-type backlight unit than the liquid crystal display device adopting the edge-type backlight unit.
In the liquid crystal display device adopting the direct-type backlight unit, the diffusion plate is manufactured by forming milk-white resin, such as acryl or polycarbonate, mixed with a diffusion agent, into a plate shape by extruding molding using rolls and printing a pattern on the extruded plate.
Even though the above-described diffusion plate is disposed in the liquid crystal display device, however, a brightness difference occurs between regions where the lamps are disposed and remaining regions where the lamps are not disposed, with the result that it is possible to accomplish uniform brightness throughout the backlight unit and the liquid crystal display device.
Furthermore, even though the above-described diffusion plate is disposed in the liquid crystal display device, the shapes of the lamps, disposed below the diffusion plate, appear on the top, i.e., bright lines appear on the top, with the result that the quality in appearance of the backlight unit and the liquid crystal display device deteriorates.